The purpose of this project is two-fold. First, the basic cardiac muscle model as it applies to isolated, isometric rat myocardium is to be extended and quantified in order to define the role and relative importance of each of the elements comprising the normal adult myocardium. Concurrently, the age-associated changes in each of these structural elements are to be defined. The second element involves the role of the dynamic mechanical properties in the transduction of the metabolic process of contraction into externally manifest work. Here, we are concerned not only with the effect of an exogenous agent on the metabolic machinery, but also with its effect on the inherent stiffness-determining properties of the muscle. It is possible that the two sites (metabolic and structural) may be affected diametrically by a given agent studied as a function of age; this could account for the relatively unimpressive age changes observed to date in isolated myocardial preparations.